Atoms and Molecules Class 9 Notes: CBSE Science Chapter 3

Law of Conservation of Mass:

• In a chemical action, the law of conservation of mass stipulates that mass can not be created or destroyed.

• According to this law, the overall mass of the products remains equal to the total mass of the reactants after any physical or chemical change.

Law of Constant proportion:

• This law was expressed by another French chemist, Joseph Proust, as follows: A chemical compound always comprises the same elements mixed in the same proportion by mass.

Law of Multiple Proportions:

• As established by John Dalton, when two elements combine to form two or more compounds, the mass of the element that combines with the fixed mass of the other bears a simple whole-number ratio $\left( \text{1803} \right)$.

Dalton’s Atomic Theory:

• According to Dalton's atomic theory, all matter, whether an element, a compound, or a mixture, is made up of microscopic particles called atoms.

• This theory's postulates are as follows::

1. All matter is made up of atoms, which are very small tiny particles that engage in chemical reactions.

2. In a chemical reaction, atoms are indivisible particles that cannot be formed or destroyed.

3. A given element's atoms have the same mass and chemical characteristics.

4. The masses and chemical characteristics of atoms of various elements differ.

5. Compounds are formed when atoms join in a ratio of tiny whole numbers.

6. In a given compound, the number and types of atoms remain constant.

Atom:

• An atom is an element's defining structure that can't be broken chemically.

• The electron, proton, and neutron are the three particles that make up an atom.

• An atom's nucleus holds the entire mass of the atom.

• An atom's electrons are grouped in shells/orbitals.

• The atomic symbol is made up of three parts: 

• The symbol $\text{X}$ - standard element symbol; 

• The atomic number $\text{A}$- represents the number of protons; 

• The mass number $\text{Z}$ - represents the total amount of protons and neutrons in an element.

• The radius of an atom is measured in nanometres.

Atomic Mass:

• The atomic mass was proposed by Dalton as an atomic hypothesis.

• The average mass of an atom, or a set of atoms, is the sum of the masses of the electrons, neutrons, and protons.

• The atomic mass is the mass of an atomic particle.

• This is often stated in terms of a unified atomic mass unit, as per the international agreement (AMU).

• The average mass of one atom of an element, as compared to $\dfrac{1}{12}$ th the mass of one carbon- $12$ atom, is called atomic mass.

Valency:

• The electrons in the atom's outermost orbit are referred to as valence electrons.

• The valency of an atom is determined by its ability to lose, gain, or share valence electrons in order to complete its octet.

Molecule:

• The total masses of the elements present in a molecule is the molecule's molecular mass.

• The atomic mass of an element is multiplied by the number of atoms in the molecule, and the masses of all the elements in the molecule are added to get the molecule's mass.

• The number of atoms in a single molecule of an element is known as its atomicity.

• For example, each of the molecules of hydrogen, nitrogen, oxygen, chlorine, iodine, and bromine has two atoms, and hence they all have two atomicities each.

Compound:

• When two or more elements join chemically in a defined mass ratio, the result is known as a compound.

• Compounds are substances made up of two or more different types of atoms in a specific ratio.

Ions:

• An ion is an atom or molecule with a net positive or negative charge due to the gain or loss of one or more of its valence electrons.

• A negatively charged particle is an anion, and a positively charged particle is a cation.

• Ionic compounds are chemical compounds in which ions are held together by ionic bonds, which are a type of specialised bond.

• The positive and negative charges in an ionic substance are always in equal amounts.

Molecular Mass:

• The total masses of the elements present in a molecule is known as the molecule's molecular mass.

• The atomic mass of an element is multiplied by the number of atoms in the molecule, and then the masses of all the elements in the molecule are added.

Mole and Avogadro Number:

• A mole is the number of entities existing in a substance, such as atoms, molecules, and ions.

• A mole is \[\text{6}\text{.022 }\times\text{ 1}{{\text{0}}^{\text{23}}}\] molecules of any substance.

• One of the most practical ways of describing the number of reactants and products in a reaction is to use the mole idea.

• Avogadro's number has a value of about \[\text{6}\text{.022 }\times\text{ 1}{{\text{0}}^{\text{23}}}\].

• Avogadro's number is a formula that calculates the number of particles in one mole (or mol) of a substance.

• It's possible that these particles are electrons, molecules, or atoms.

• No. of Moles can be calculated as;

$\text{1 Mole = }\dfrac{\text{Mass of a substance}}{\text{Gram atomic mass}}$

$\text{n = }\dfrac{\text{Molecular Weight}}{\text{Empirical Formula Weight}}$

$\text{n = }\dfrac{\text{Given No}\text{. of particles}}{\text{Avogadro Number}}$

$\text{n = }\dfrac{\text{N}}{{{\text{N}}_{A}}}$

Some Important formulae:

• \[\text{No}\text{. of Molecules = }\dfrac{\text{Given Mass}}{\text{Molar Mass}}\text{  }\times\text{  Avogadro Number}\]

Hence,

$\text{N = }\dfrac{\text{m}}{\text{M}}\text{  }\times\text{  }{{\text{N}}_{\text{A}}}$

• $\text{1 Mole = }\dfrac{\text{Mass of a substance}}{\text{Gram atomic mass}}$

• $\text{Mass of a substance = }\dfrac{\text{Molar Mass of a substance}}{\text{No}\text{. of Moles}}$

• $\text{Percentage composition of an Element = }\dfrac{\text{Total weight of element in a molecule}}{\text{Gram Molecular Weight}}\times 100$

• $\text{RMM = }\dfrac{\text{Mass of one molecule of the substance}}{\left( \dfrac{\text{1}}{\text{12}} \right)\text{ Mass of the atom of Carbon }\left( {{\text{C}}^{\text{12}}} \right)}$

• $\text{Gram Molecular Volume = }\dfrac{\text{Gram Molecular Weight}}{\text{Weight / Volume of gas at STP}}$

Important Topics of Class 9 Science Chapter 3 Atoms and Molecules you shouldn’t Miss!

These topics provide a comprehensive foundation for understanding the nature of matter and the principles of chemical reactions, crucial for further studies in chemistry.

1. Atomic Theory: Understanding the basic structure of an atom, including protons, neutrons, and electrons, and how they contribute to the atom's properties.

2. Molecular Structure: Learning about how atoms bond together to form molecules and the types of chemical bonds, such as ionic and covalent bonds.

3. Chemical Formulas: Familiarise yourself with how chemical formulas represent molecules and compounds, including the notation for elements and the concept of valency.

4. Laws of Chemical Combination: Exploring fundamental laws such as the Law of Conservation of Mass and the Law of Definite Proportions, which describe how atoms and molecules interact and combine.

5. Mole Concept: Understanding the concept of a mole, Avogadro’s number, and how it relates to the quantity of particles in a substance.

Importance of Chapter 3 Atoms and Molecules Class 9 Notes

Atoms and Molecules Class 9 Notes PDF provides a crucial understanding of the basic building blocks of matter, which is essential for further studies in chemistry and for applying chemical principles in practical scenarios.

1. Foundation for Chemistry: This chapter introduces the basic concepts of atoms and molecules, which are essential for understanding more advanced topics in chemistry and chemical reactions.

2. Understanding Molecular Structure: It helps you grasp how atoms combine to form molecules and compounds, which is crucial for studying chemical reactions and interactions.

3. Chemical Bonding: Knowledge of ionic and covalent bonds is fundamental for understanding how substances react and form new compounds, laying the groundwork for more complex chemical principles.

4. Chemical Formulas and Equations: Learning how to write and interpret chemical formulas and equations is key for solving problems and conducting experiments in chemistry.

5. Principles of Chemical Reactions: This chapter covers the principles governing chemical reactions, which are vital for predicting how substances will interact and transform in various processes.

Tips for Learning the Class 9 Science Chapter 3 Atoms and Molecules

Here are some tips for learning Class 9 Science Chapter 3: Atoms and Molecules:

• Grasp Basic Concepts: Start by understanding the fundamental structure of atoms, including protons, neutrons, and electrons. Knowing how these particles interact is crucial for understanding molecular structure.

• Use Visual Aids: Draw diagrams of atomic structures and molecular formations to visualize how atoms bond together. This can help you better understand complex concepts.

• Learn Chemical Bonding: Focus on the types of chemical bonds—ionic and covalent—and understand how they form and affect molecular structure. Practice drawing Lewis structures to represent these bonds.

• Practice Writing Formulas: Practice writing and interpreting chemical formulas and equations. This helps in understanding how molecules are represented and how chemical reactions are balanced.

• Study Chemical Laws: Familiarize yourself with key laws such as the Law of Conservation of Mass and the Law of Definite Proportions. These laws explain how substances combine and react.

Conclusion

Chapter 3 of Class 9 Science, Atoms and Molecules, is crucial for building a solid understanding of the fundamental components of matter. By mastering the structure of atoms, types of chemical bonds, and the principles of molecular formation, you lay the groundwork for more advanced chemistry topics. Utilising visual aids, practising chemical formulas, and understanding key chemical laws will enhance your grasp of these concepts. Use these notes to review and reinforce your knowledge, preparing you effectively for exams and further studies in chemistry.

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